Conventionally, one of air-fuel ratio control apparatuses of the type comprises an upstream air-fuel ratio sensor, a catalytic converter, and a downstream air-fuel ratio sensor, disposed in this order from an upstream side to a downstream side in an exhaust passage of an engine, and is configured to perform a feedback control on an air-fuel ratio (hereinafter, simply referred to as “an air-fuel ratio of the engine”) of a mixture supplied to the engine, based on an output value of the upstream air-fuel ratio sensor and an output value of the downstream air-fuel ratio sensor.
More specifically, the conventional air-fuel ratio control apparatus (the conventional apparatus) calculates a sub feedback amount (a first feedback amount) to have the output value of the downstream air-fuel ratio sensor coincide with (becomes equal to) a target downstream side value (for example, a value corresponding to the stoichiometric air-fuel ratio), by performing a proportional-integral processing on an error (difference) between the output value of the downstream air-fuel ratio sensor and the target downstream side value.
Further, the conventional apparatus calculates a main feedback amount to have the air-fuel ratio of the engine coincide with (becomes equal to) a target upstream air-fuel ratio (for example, the stoichiometric air-fuel ratio), based on the output value of the upstream air-fuel ratio sensor and the sub feedback amount. Thereafter, the conventional apparatus performs the feedback control on the air-fuel ratio of the engine (for example, a fuel injection amount) based on the calculated main feedback amount.
It should be noted that, in the present specification, performing a main feedback control means newly calculating (or updating) the main feedback amount, and using the main feedback amount for the control of the air-fuel ratio of the engine. Similarly, performing a sub feedback control means newly calculating (or updating) the sub feedback amount, and using the sub feedback amount for the control of the air-fuel ratio of the engine.
Meanwhile, when the sub feedback control has been performed for an adequately long time, the sub feedback amount converges on (comes close to) a certain value. The certain value is referred to as a convergence value. The convergence value indicates (or represents) a degree of a difference between an average of an air-fuel ratio of a gas flowing into the catalytic converter and the target downstream air-fuel ratio. In other words, the sub feedback amount converges on the convergence value that is affected by an error in measuring an air amount by an air-flow meter, an error in a fuel injection amount due to an injection property (characteristic) of a fuel injector, and an error in detecting the air-fuel ratio by the upstream air-fuel ratio sensor, and the like (hereinafter, these errors are referred to as “an intake-exhaust relating error”.
Accordingly, for example, in a period before the downstream air-fuel ratio sensor is activated, or in a period from a timing at which the sub feedback control is started when the downstream air-fuel ratio sensor is activated to a timing at which the sub feedback amount reaches a value close to the convergence value, it is preferable that the air-fuel ratio of the engine be controlled using the convergence value of the sub feedback amount which was obtained in a previous operation of the engine.
In view of the above, the conventional apparatus performs a “learning (control)” in which a learning value is updated based on a “value according to the calculated sub feedback amount” while the sub feedback control is being performed. The “value according to the calculated sub feedback amount” is, for example, a “value according to a steady-state (stationary) component included in the sub feedback amount”, such as an “integral term and/or a proportional term” which are/is a resultant value(s) of the proportional-integral processing.
The learning value is stored in a backup RAM (a stand-by RAM) included in the conventional apparatus, or in a nonvolatile memory such as an EEPROM. An electrical power is supplied to the backup RAM regardless of a position of an ignition key switch of a vehicle on which the engine is mounted. The backup RAM can retain (hold) “stored values (data)” as long as it is supplied with the electrical power from the battery. The conventional apparatus performs the control of the air-fuel ratio of the engine using the learning value.
According to the configuration described above, it is possible to compensate for an error (or a deviation) of the sub feedback amount from the convergence value by the learning value. That is, even when the sub feedback amount deviates from the convergence value before or immediately after the start of the sub feedback control, the deviation can be compensated by the learning value. As a result, the air-fuel ratio of the engine can be controlled in such a manner that it is always close to an appropriate value.
However, for example, when the electrical power supply “from the battery to the backup RAM” is stopped, such as when the battery is removed from the vehicle, and when the battery is completely discharged, the learning value stored in the backup RAM is lost (eliminated, broken). Further, the learning value stored in the backup RAM or the nonvolatile memory may be destroyed due to some electrical noise, or the like. In these cases, the learning value is set (returned) to an initial value (a default), and therefore, it is preferable to have the learning value come close to the convergence value in a short time (i.e., the learning be completed in a short time).
in view of the above, an air-fuel ratio control apparatus disclosed in Japanese Patent Application Laid-Open (kokai) No. Hei 5-44559 sets a changing/updating amount of the learning value (i.e., a changing speed of the learning value) to (at) a larger value after the learning value is set/returned to the initial value, or the like, to thereby have the learning value come closer to the convergence value in a short time (promptly). Accordingly, a period can be shortened in which “the air-fuel ratio of the engine deviates from the appropriate value due to an insufficient compensation for the intake-exhaust error, and thus, the emission becomes worse”. It should be noted that this type of the “control for having the learning value come closer to the convergence value in a short time” is referred to as “an expedited (facilitated, accelerated) learning control”.